1. Field of the Invention
The present invention relates to a graphite crucible used in the manufacturing of a silicon single crystal according to the Czochralski method. It further relates to a silicon single crystal manufacturing apparatus according to the Czochralski method, comprising the graphite crucible.
2. Background Art
As one method for growing silicon single crystals, pulling methods represented by the Czochralski method (CZ method) have been generally used. Normally, an apparatus used therein has a quartz crucible for retaining the raw material silicon melt, the crucible being surrounded by a graphite crucible having an inner shape that serves to support the quartz crucible and to achieve uniform heating, and a heater for heating the melt being arranged outside thereof. Usually, the shapes of both the quartz crucible and the graphite crucible consist of a side wall having an approximately cylindrical shape and an appropriately radially chamfered bottom portion.
In the CZ method, a silicon raw material placed in the quartz crucible is heated and melted. Since the melting point of silicon is about 1420° C., heating first has to be carried out so as to reach the melting point of silicon. In this case, the temperature of the heater has to be increased to about 1700° C., and as a result, both the graphite crucible and the quartz crucible are heated to the melting point of silicon or higher. When quartz exceeds about 1200° C., it begins to soften and deform, and, due to the load of the molten silicon in the quartz crucible, the quartz crucible comes almost entirely into close contact with the inner shape of the graphite crucible supporting the quartz crucible from the outside. Along with a recent increase in the diameter of silicon wafers, an increase in the diameter and capacity of quartz crucibles is required in the CZ method as well, and it is becoming more important to control the temperature range and heating uniformity of the heating process.
As shown in the equations below, a known problem of the CZ method is that, at the silicon melting temperature, a reaction is generated on the inner surface of the quartz crucible between SiO2, which is a component of the quartz crucible, and the molten silicon Si. As a result, an SiO gas is generated. Furthermore, outside of the quartz crucible, this SiO gas reacts with the outer surface of the graphite member, thereby forming an SiC solid. Moreover, on the outer surface of the quartz crucible, a SiO gas and a CO gas are generated by a reaction between the outer surface of the quartz crucible and the inner surface of the graphite crucible, and further SiC solids are formed by reaction with the inner surface of the graphite crucible. The SiC thus formed has a thermal expansion coefficient significantly different from that of graphite. Therefore, SiC becomes a cause of cracks, etc. during the cooling/heating cycle of the graphite crucible, thus limiting its service life in terms of safety, etc. Moreover, CO gas generated during this process applies a pressure to the quartz crucible, causing it to deform.SiO2+Si→SiOSiO2+C→SiO+COSiO+2C→SiC+CO
Several techniques directed to improving the graphite crucible in order to solve the above problems are known. One of them is a method disclosed in Japanese Patent Application Laid Open No. 61-44791, in which the CO gas generated between the quartz crucible and the graphite crucible is discharged by flowing an inert gas such as argon in a downstream direction between the quartz crucible and the graphite crucible and discharging the gas through holes which are provided with a downward direction in a lower part of the side surface of the graphite crucible. Moreover, a technique in which horizontal holes are provided in the graphite crucible at positions above the liquid level of the molten silicon in order to prevent deformation of the quartz crucible caused by the generated gas is disclosed in Japanese Patent Application Laid Open No. 10-297992. Furthermore, a method in which the CO gas generated between the quartz crucible and the graphite crucible is discharged by flowing an inert gas such as argon in a downstream direction toward a bottom portion of the crucible along gas guiding grooves which are provided vertically on the inner surface of the graphite crucible and discharging the gas from the bottom portion is disclosed in Japanese Patent Application Laid Open No. 2008-201619.
However, it has now begun to be recognized that these conventional methods are no longer adequate for the large-diameter/large-capacity (high weight) quartz crucibles which are, particularly recently, used in the CZ method. More specifically, providing complicated structures, such as gas guiding grooves, on the inner surface of the graphite crucible in order to discharge the generated gas is not preferred in terms of safe retention of the large-capacity (high weight) quartz crucible. Moreover, when a quartz crucible having a large diameter/large capacity (high weight) is used, the quartz crucible softened due to the high heating temperature deforms so that it almost entirely comes into close contact with the graphite crucible supporting it. Thus, it becomes difficult to provide a sufficient gas flow between the crucibles.
Therefore, novel graphite crucible technologies which prevent accumulation of pressure on the quartz crucible by smoothly discharging the generated CO gas and also prevent formation/accumulation of SiC on the inside of the graphite crucible, both without (i) providing a complicated structure on the inner surface of the graphite crucible and without (ii) providing a sufficient inert gas flow between the graphite crucible and the quartz crucible, are desired.